1. Field of the Invention
The present invention relates generally to fuel assemblies for nuclear reactors and, more particulary, is concerned with an apparatus for repairing a grid spring which protrudes outwardly from the outer strap of the grid without applying an impact force thereto.
2. Description of the Prior Art
In most nuclear reactors, the reactor core is comprised of a large number of elongated fuel assemblies. Conventional designs of these fuel assemblies include a plurality of fuel rods held in an organized array by a plurality of grids spaced axially along the fuel assembly length and attached to a fuel assembly. Top and bottom nozzles on opposite ends of the fuel assembly are secured to the guide thimbles which extend slightly above and below the ends of the fuel rods.
The grids as well known in the art are used to precisely maintain the spacing between the fuel rods in the reactor core, prevent rod vibration, provide lateral support for the fuel rods, and, to some extent, frictionally retain the rods against longitudinal movement. Conventional designs of grids include a multiplicity of interleaved inner straps having an egg-crate configuration designed to form cells which individually accept the fuel rods and control rod guide thimbles. The cells of each grid which accept and support the fueld rods at a given axial location theralong typically use relatively resilient springs and relatively rigid protrusions (called dimples) formed into the metal of the interleaved straps. The springs and dimples of each grid cell frictionally engage or contact the respective fuel rod extending through the cell. Additionally, outer straps are attached together and peripherally enclose the inner straps to impart strength and rigidity to the grid. The outer straps conventionally have springs integrally formed into the metal thereof which project into respective ones of the cells disposed along the perimeter of the grid.
In view that the operation of the reactor core involves irradiation of the fuel rods, it is desirable that the supporting forces imposed by the grid on the fuel rods be such that the grid adequately supports the fuel rods but does not promote lengthwise distortion thereof over time. Also, in view that the grids are irradiated along with the fuel rods they support, the grids inherently degrade somewhat over time as a result. In particular, notwithstanding improvements in grid construction, such as illustrated and described in U.S. Pat. No. 4,474,730 assigned to the assignee of the present invention, over the life of the fuel assembly the metal of the interleaved straps forming the grid, and thereby the springs and dimples formed therein, are subject to stress relaxation due to irradiation.
In light of the above conditions, occasionally a few of the grid springs formed in the outer strap of the grid become deformed and portions thereof protrude outwardly from the grid. Since it is desirable to reposition such springs to enhance grid function, protruding grid springs are one of the many items watched for and repaired when discovered during periodic inspection of the fuel assemblies. The presently qualified procedure for repairing protruding grid springs uses a plastic mallet and an adjustment block and requires that a light impact force be applied against the grid to the fuel bundle. Not only does this procedure entail shortcomings from the standpoint of the undesirability of applying even light impacts against the fuel assembly in doing repair work, it also has shortcomings in terms of the lack of uniformity and standardization of the repair results that can be achieved by following it.
Consequently, a need exists for a technique to replace the present procedure of protruding grid spring repair which will achieve its objective in a more uniform and repeatable fashion and without having to impose even light impacts on the fuel assembly.